Ceramic sintered bodies and a process for manufacturing the same

ABSTRACT

High density fine ceramic sintered bodies are disclosed, which have a maximum pore diameter of not more than 10 μm and a porosity of not more than 0.5%. A process for producing such high density fine ceramic sintered bodies comprises the steps of mixing a ceramic raw material powder with a sintering aid, grinding the resulting mixture granulating and shaping the mixture, and firing the shaped body. The granulated powder is once forcedly dried, and upon necessity is added with water and/or sieved to obtain a uniform granulated powder having a given amount of water.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to high density fine ceramic sinteredbodies useful for engine parts, gas turbine parts, mechanical parts,wear resistant slide members, and the like, and a process formanufacturing the same.

(2) Related Art Statement

In order to produce ceramic products, it is a conventional practice, asshown in a flow chart of FIG. 5 by way of an example, a ceramic rawmaterial is first mixed with a sintering aid, and the mixture is groundand passed through a sieve of 44 μm to remove foreign matters such asbroken pieces of grinding media used for the grinding. Then, aftergranulating, water is added to the granulated powder as necessary, andthe granulated powder is shaped by a mold press or a cold isostaticpress. The shaped body is finally sintered at a given temperature toobtain a sintered product.

However, since a positive measure is not taken to uniformly dispersewater in the granulated powder in the above-mentioned conventionalceramic product-producing process, an amount of water locally varies inthe granulated powder. As a result, pores are formed in shaped bodiesdue to non-uniform particle fracture which is caused by non-uniformwater distribution in the granulated powder, so that such pores remainin the sintered products. Consequently, ceramic sintered bodies havingexcellent mechanical characteristics cannot be obtained.

Particularly, when ceramic sintered bodies are used as bearing members,wear resistant members or slide members, pores and hardness largelyinfluences the use life thereof. Thus, in order to obtain ceramicproducts having longer use life than before, it was necessary to producehigh hardness ceramic sintered products having a smaller pore diameterand a smaller porosity. Among them, when they are used as bearingmaterials, it is known that it's important to grasp rolling fatigue lifeof the materials. Thus, there has been a demand to develop dense, highstrength, and/or high hardness ceramic materials to improve rollingfatigue life.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate the above-mentioneddrawbacks, and to provide dense ceramic sintered bodies having highstrength and/or high hardness, as well as a process for producing thesame.

The ceramic sintered bodies according to the present invention arecharacterized in that the maximum pore diameter is not more than 10 μm,and the porosity is not more than 0.5%. Preferably, four point flexuralstrength at room temperature is not less than 100 kg/mm² and/or Knoophardness is not less than 15.5 GPa.

The process for producing the ceramic sintered bodies according to thepresent invention comprises the steps of mixing a ceramic raw materialpowder with a given sintering aid, grinding, granulating, and shapingthe resulting mixture, and firing the shaped body, and is characterizedin that after the granulation, uniform granulated powder having a givenamount of water is obtained by forcedly drying the granulated powder andthen, as necessary adding water to the powder and/or sieving the powderthrough a sieve.

In order to obtain silicon nitride sintered bodies, it is preferablethat a silicon nitride raw material containing not less than 90% byweight of α-silicon nitride is used and that firing is effectedpressurelessly (i.e. under atmospheric pressure). More preferably, anobtained shaped body is preliminarily treated, and then treated by a hotisostatic press (HIP) in a nitrogen atmosphere.

In the above construction, a uniform granulated powder having novariation in the content of water over the granulated particles can beobtained by once forcedly drying the granulated powder, and at need,adding water and/or passing the granulated powder through a sieve.

That is, pores present between the particles can be reduced by attaininga uniformly press-crushed state during shaping through forcedly dryingthe granulated powder and adding water thereto at need. As a result,when the thus obtained granulated powder is shaped and fired, forinstance, as to silicon nitride sintered bodies, high strength and highdensity ceramic sintered bodies having the maximum pore diameter of notmore than 10 μm, porosity of not more than 0.5%, four point flexuralstrength of not less than 100 kg/mm² at room temperature, and Knoophardness of not less than 15.5 GPa can be obtained even by pressurelesssintering.

These and other objects, features and advantages of the presentinvention will be appreciated upon reading of the following descriptionof the invention when taken in conjunction with the attached drawings,with the understanding that some modifications, variations, and changesof the same could be made by the skilled person in the art to which theinvention pertains without departing from the spirit of the invention orthe scope of claims appended hereto.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

For a better understanding of the invention, reference is made to theattached drawings, wherein:

FIG. 1 is a flow chart illustrating an example of the producing processaccording to the present invention;

FIG. 2 is a graph showing the relation between the four point flexuralstrength of sintered bodies according to the present invention and thegranulated powder-drying temperature;

FIG. 3 is a graph showing the relation between the four point flexuralstrength of sintered bodies according to the present invention and theamount of water added to the granulated powder;

FIG. 4 is a graph showing the relation between the four point flexuralstrength of the sintered bodies according to the present invention andthe sieve opening after grinding; and

FIG. 5 is an example of a flow chart of a conventional process for theproduction of a silicon nitride sintered bodies.

DETAILED DESCRIPTION OF THE INVENTION

In order to obtain silicon nitride sintered bodies according to thepresent invention, any sintering aid may be used so long as it can makethe silicon nitride dense, strong and hard. However, it is preferable touse MgO, ZrO₂, Y₂ O₃ and/or a compound of Mg, Zr or Y which is convertedto MgO, ZrO₂, or Y₂ O₃ by heating, respectively. The reason is that MgO,ZrO₂, Y₂ O₃ or the Mg-, Zr- or Y-compound promotes a phasetransformation to an elongated or rod-like β-silicon nitride crystalswhich are advantageous for high strengthening and the Zr compoundstrengthens an intergranular phase when present in the intergranularphase during the sintering. The reason why it is preferable that the Mgcompound is added in an amount of from 0.5 to 15% by weight whencalculated as MgO and that the Zr compound is added in an amount of from0.5 to 13% by weight when calculated as ZrO₂ and that Y compound isadded in an amount of from 2 to 15% by weight when calculated as Y₂ O₃,is that if they fall outside these respective ranges, theabove-mentioned effects are reduced. Further, in the case of siliconnitride pressurelessly sintered bodies, it is preferable that not lessthan 90% by weight of silicon nitride is β-silicon nitride crystals.This is because if it is less than 90% by weight, it is difficult toattain high strength.

The forcedly drying temperature is preferably in a range of from 60° to100° C. The reason is that if it is less than 60° C., it is difficult toattain a desired dried state, while if it is over 100° C., it is alsodifficult to attain a uniform press-crushed state of the granulatedpowder due to hardening of an auxiliary used in spray drying.

Furthermore, it is preferable that the ground raw material is passedthrough a sieve having a sieve opening of not more than 32 μm beforegranulating or the forcedly dried and water-added granulated powder ispassed through a sieve having a sieve opening of not more than 250 μm.The reason is that coarse particles after the grinding and foreignmatters contained in the raw materials cannot effectively be removed byusing a sieve having a larger sieve opening than the above, so that itis difficult to maintain uniformity of the graulated powder.

In addition, the amount of water added to the granulated powder ispreferably in a range from 0.5 to 5% by weight. The reason is that if itis less than 0.5% by weight, water may not be uniformly distributedamong the granulated particles to cause variation in the water content,while if it is over 5% by weight, water may ooze out from the surface ofthe shaped body during shaping to cause non-uniform water distributionin the shaped body.

The granulation is preferably effected by spray drying. The reason isthat granulated powder of which packing density can be increased duringthe shaping can be obtained thereby.

Polyvinyl alcohol (PVA), polyethylene glycol (PEG), methyl cellulose(MC), and stearic acid are preferred as an auxiliary used in the spraydrying. The reason is that the granulated powder is difficult to hardenor break by forcedly drying it and/or adding water thereto when such asauxiliary is used.

It takes a long time to mix and grind a raw powder having an averageparticle size of more than 2 μm. During such a long time mixing andgrinding, there is a possibility that impurities enter the raw materialdue to wearing during the grinding so that characteristics of thesintered body are deteriorated and an effect of densifying the sinteredbody is lost. Thus, it is preferably to use fine raw materials havingthe average particle size of not more than 2 μm and more preferably notmore than 1 μm.

When the thus obtained shaped body is preliminarily treated and isfurther treated by hot isostatic press (HIP) in an inert gas atmosphere,higher density, higher strength, and/or higher hardness can be attained.Such treatments are preferable because, for instance, the maximum porediameter of not more than 10 μm and the porosity of not more than 0.3%can be attained. As an inert gas, nitrogen, argon, or the like is used.

Furthermore, it is preferable to use a silicon nitride raw materialcontaining not less than 90%, more preferably, not less than 95% ofα-silicon nitride in the production of the silicon nitride sinteredbodies, because a sufficient amount of needle-like β-silicon nitridecrystals which lead to high strength during sintering are precipitatedowing to the α→β transformation.

In a HIP sintered body as a preferred embodiment according to thepresent invention, steps of forcedly drying, shaping, preliminarilytreating and hot isostatic press treatment are effected in this order.In this process, the preliminary treatment is broken down into twokinds: a primary sintering method in which the shaped body is primarilyfired, and an encapsulating method in which a shaped body is sealed in acapsule. In the primary sintering method of the preliminary treatment,the shaped body is primarily fired, preferably at 1,400° to 1,600° C. inan inert gas atmosphere at ordinary pressure. If the firing temperatureis less than 1,400° C., open pores do not disappear even after thefiring, so that dense sintered bodies cannot be obtained even after thehot isostatic press treatment. On the other hand, if it is more than1,600° C., a decomposition reaction proceeds during sintering, so thatdense, high strength, and/or high hardness sintered bodies cannot beobtained even after the HIP treatment, either.

Meanwhile, there are two methods for the encapsulating step. That is, ashaped body is sealed in a vacuum-evacuated glass vessel preferablymainly consisting of SiO₂ before the HIP treatment. Alternatively, ashaped body is buried in a glass powder before the HIP, and the glasspowder is melted as the temperature rises in the HIP treatment toencapsulate the shaped body. The reason why glass is preferred as anencapsulating material is that glass has more excellent deformingability and sealability as a capsule during the hot isostatic presstreatment.

FIG. 1 shows a flow chart of an example of the producing processaccording to the present invention. First, a ceramic raw material havingthe average particle size of not more than 2 μm is mixed and ground witha sintering aid, and the thus obtained mixture is passed through a sievehaving a sieve opening of, preferably, not more than 32 μm to removeforeign matters and coarse particles such as broken pieces of grindingmedia used for grinding. Any sintering aid may be used as long as it candensify and strengthen intended ceramic materials. Then, after themixture is granulated to obtain a granulated powder containing around 1%by weight of water, the granulated powder is sieved similarly in aconventional manner. The resulting granulated powder is forcedly dried,preferably, at a temperature range from 60° to 100° C. to obtain auniform granulated powder containing water at a low variation in anamount of from 0.2 to 0.5% by weight. Next, 0.5 to 5.0% by weight ofwater is added to the granulated powder as necessary to obtain thegranulated powder having a uniform water content, and a final granulatedpowder is obtained by removing coarse particles coagulated by theaddition of water, with a sieve having a sieve opening of not more than250 μm. High strength, high density and/or high hardness ceramicsintered bodies having the characteristics of the present invention areobtained by shaping the thus obtained granulated powder in an ordinarymanner and firing the resulting shaped body at ordinary pressure.

In the following, examples of the present invention will be explained.These examples are merely given in illustration of the invention, butshould never be interpreted to limit the scope thereof.

EXAMPLE 1

Into 100 parts by weight of a Si₃ N₄ powder having the average particlesize of 0.5 μm were added and mixed 3 parts by weight of MgO, 1 part byweight of ZrO₂, 4 parts by weight of CeO₂, and 1 part by weight of SrOas sintering aids. After 60% by weight of water was added to the thusobtained mixture together with grinding media having a diameter of 5 to10 mm, the mixture was mixed and ground for 4 hours by a batch grinder.

Next, the mixed and ground slurry was passed through a JIS standardsieve of a sieve opening of 32 μm, which was added and mixed with 2% byweight of PVA and 0.2% by weight of stearic acid used as auxiliaries inspray drying. Thereafter, a granulated powder having an average particlesize of 80 μm and a water content of from 0.5 to 1.0% by weight wasobtained by spray-drying.

Further, the granulated powder was forcedly dried at a dryingtemperature shown in Table 1 for 24 hours by using a thermoplasticdrier, and water was added thereto, as necessary. When water was added,the powder was passed through a JIS standard sieve having a sieveopening shown in Table 1, thus obtaining granulated powders (Sample Nos.1 to 8). The granulated powder was shaped at a pressure of 3 ton/cm² bycold isostatic press, thereby obtaining a shaped body of 60 mm×60 mm×6mm.

Then, after the shaped body was dewaxed at a temperature of 500° C. for3 hours, it was pressurelessly sintered at a temperature of 1,700° C.for 1 hour in a nitrogen gas atmosphere, thus obtaining high strengthsilicon nitride sintered bodies according to the present invention(Sample Nos. 1 to 8). Apart from the above, granulated powders in SampleNos. 9-11 were prepared as Comparative Examples of the present inventionunder producing conditions shown in Table 1 with no forcedly drying, andwere shaped and fired under similar conditions, thereby obtainingsintered bodies.

Then, with respect to the thus obtained sintered bodies, flexuralstrength, maximum pore diameter, and porosity were measured, andmeasurement results are shown in Table 1. The flexural strength wasmeasured according to a four point flexural strength test method of afine ceramic flexural strength-testing in JIS R-1601. The maximum porediameter and the porosity were measured with respect to amirror-polished surface of the sintered body by means of an opticalmicroscope at a 400 x magnification. The maximum width of a pore wastaken as the diameter of the pore, and the maximum diameter of 1,000pores measured was taken as the maximum pore diameter. With respect tothe porosity, the total pore area was determined by actually measuringareas of the above 1,000 pores in the above measurement, and theporosity was determined as a value obtained by dividing the total porearea by a total area as viewed in the measurement.

                                      TABLE 1                                     __________________________________________________________________________            Producing conditions  Measurement results                                     Forcedly      Sieve opening Maximum                                           drying Amount of                                                                            after water                                                                           Flexural                                                                            pore                                              temperature                                                                          water added                                                                          added   strength                                                                            diameter                                                                            Porosity                            Sample No.                                                                            (°C.)                                                                         (wt %) (μm) (kg/mm.sup.2)                                                                       (μm)                                                                             (%)                                 __________________________________________________________________________    Present                                                                            1  45     --     --      91    10    0.5                                 inven-                                                                             2  45     3      325     95    7     0.3                                 tion 3  60     0.5    250     99    7     0.2                                      4  80     3      149     113   6     0.2                                      5  80     5      250     104   7     0.2                                      6  100    --     --      95    8     0.4                                      7  120    3      149     97    7     0.4                                      8  140    6      325     92    9     0.3                                 Compar-                                                                            9  --     --     --      81    23    2.7                                 ative                                                                              10 --     5      325     85    18    0.8                                 example                                                                            11 --     3      --      79    32    3.6                                 __________________________________________________________________________

As compared with Comparative Examples, it is clear from Table 1 that thesintered bodies according to the present invention using a mixed rawmaterial having been forcedly dried, and added with water and sieved atneed, are less porous, superior sintered bodies having extremely higherstrength.

EXAMPLE 2

Into a pot made of zirconia were placed 100 parts by weight of ZrO₂powder having the average particle size of 1.8 μm, 5 parts by weight ofa stabilizer Y₂ O₃, 2 parts by weight of a sintering aid Al₂ O₃, and 100parts by weight of water, which were mixed and ground. As shown in Table2, the grinding was effected for 1 hour, 10 hours, or 30 hours. Then, 1%by weight of an auxiliary PEG to be used in spray drying was added tothe thus mixed and ground slurry, which was spray dried to prepare agranulated powder. The thus obtained granulated powder was sampled, andwas forcedly dried at a temperature shown in Table 2 for 30 hours byusing a dry air drier. After water was added at need, the granulatedpowder was shaped at 1.5 ton/cm² by a cold isostatic press machine toobtain a shaped body of 60×60×6 mm. Thereafter, the shaped body wasdried, dewaxed, and fired at 1,400° C. in air, thus obtaining sinteredbodies according to the present invention (Sample Nos. 12 to 18).Sintered bodies (Sample Nos. 19 to 21) were obtained as ComparativeExamples by the same process except that no forcedly drying waseffected. Further, a sintered body (Sample No. 22) was obtained as aComparative Example by the same process except that ZrO₂ having theaverage particle size of 3 μm was used and the grinding was carried outfor 48 hours without being accompanied by forcedly drying.

Then, the flexural strength of the thus obtained sintered bodies and therelative density of the shaped bodies were measured, and are shown inTable 2. The flexural strength was measured by the same method as inExample 1, and the relative density of the shaped body was determined asfollows: ##EQU1##

As a result, it is seen from Table 2 that the invention products havehigher relative densities of the shape bodies and extremely improvedstrength. Thus, the high strength zirzonia sintered bodies were obtainedby the producing process according to the present invention. ComparativeExample 22 having the average particle size greater than 2 μm wasdeteriorated in a flexural strength.

                                      TABLE 2                                     __________________________________________________________________________                                      Relative                                               Average     Amount     density                                            Grind-                                                                            particle size                                                                             of         of   Maximum                                       ing after mixing                                                                         Forcedly                                                                           water                                                                              Flexural                                                                            shaped                                                                             pore                                          time                                                                              and grinding                                                                         drying                                                                             added                                                                              strength                                                                            body diameter                                                                            Porosity                         Sample No.                                                                           (hr)                                                                              (μm)                                                                              (°C.)                                                                       (%)  (kg/mm.sup.2)                                                                       (%)  (μm)                                                                             (%)                              __________________________________________________________________________    Present-                                                                           12                                                                               1  1.5    60   3    85    55    9    0.4                              inven-                                                                             13                                                                               1  1.5    100  --   81    54    9    0.3                              tion 14                                                                               1  1.5    120  --   79    53   10    0.4                                   15                                                                              10  0.7    60   --   90    55    8    0.4                                   16                                                                              10  0.7    80   3    97    56    5    0.2                                   17                                                                              30  0.4    60   2    95    55    7    0.3                                   18                                                                              30  0.4    100  4    93    55    7    0.3                              Compar-                                                                            19                                                                               1  1.5    --   --   73    51   15    0.8                              ative                                                                              20                                                                              10  0.7    --   --   80    51   11    0.6                              example                                                                            21                                                                              30  0.4    --   --   76    48   12    0.7                                   22                                                                              48  1.4    --   --   75    52   12    0.7                              __________________________________________________________________________

EXAMPLE 3

To α-silicon nitride powder having an average particle size of 0.5 μmwere mixed powdery MgO, ZrO₂, and Y₂ O₃ as sintering aids at rates of 4%by weight, 3% by weight, and 6% by weight, respectively. After 60% byweight of water was added to the thus obtained mixture together withgrinding media of 5 to 10 mm in diameter, the mixture was mixed andground for 4 hours in a batch grinder.

Then, the mixed and ground slurry was passed through a JIS standardsieve of a sieve opening of 32 μm, and 2% by weight of PVA and 0.2% byweight of stearic acid were added to the slurry as spray dryingauxiliaries, which was spray dried to obtain a granulated powder havingthe average particle size of 80 μm and a water content of from 0.5 to1.0% by weight.

Further, the granulated powder was forcedly dried at a temperature shownin Table 3 for 24 hours by using a thermostatic drier, and water wasadded thereto at need. When water was added, the granulated powder wassieved with a JIS standard sieve of a sieve opening shown in Table 3,thus obtaining granulated powders (Sample Nos. 31 to 38). The granulatedpowder was shaped at a pressure of 2.5 ton/cm² by cold isostatic press,thus obtaining a shaped body of 60 mm×60 mm×6 mm.

Then, after the shaped body was dewaxed at a temperature of 500° C. for3 hours, the shaped body was pressurelessly sintered at a temperature of1,700° C. in a nitrogen gas atmosphere for 1 hour, thus obtaining highstrength silicon nitride sintered bodies according to the presentinvention (Sample Nos. 31 to 38). Apart from the above, granulatedpowders of Sample Nos. 39 to 41 were prepared as Comparative Examples ofthe present invention under producing conditions shown in Table 3 withno forcedly drying, and the granulated powders were shaped and firedunder the same conditions, thereby obtaining sintered bodies.

Then, flexural strength, maximum pore diameter, and porosity of thesintered bodies and a ratio of β-silicon nitride crystals in thesintered body were measured in the same manner as in Example 1, andmeasurement results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________           Producing conditions  Measurement results                                     Forcedly      Sieve opening Maximum    Ratio of                               drying Amount of                                                                            after water                                                                           Flexural                                                                            pore       crystals                               temperature                                                                          water added                                                                          added   strength                                                                            diameter                                                                            Porosity                                                                           of β-Si.sub.3 N            Sample No.                                                                           (°C.)                                                                         (wt %) (μm) (kg/mm.sup.2)                                                                       (μm)                                                                             (%)  (%)                             __________________________________________________________________________    Present                                                                            31                                                                              45     --     --      101   9     0.5  100                             inven-                                                                             32                                                                              45     3      325     106   6     0.3  100                             tion 33                                                                              60     0.5    250     110   5.5   0.2  100                                  34                                                                              80     3      149     125   5     0.2  100                                  35                                                                              80     5      250     115   6     0.2  100                                  36                                                                              100    --     --      105   8     0.4  100                                  37                                                                              120    3      149     108   6     0.4  100                                  38                                                                              140    6      325     102   8     0.3  100                             Compar-                                                                            39                                                                              --     --     --       90   22    2.7  100                             ative                                                                              40                                                                              --     5      325      94   17    0.8  100                             example                                                                            41                                                                              --     3      --       88   31    3.6  100                             __________________________________________________________________________

As is clear from Table 3, the sintered bodies according to the presentinvention using the mixed powder being forcedly dried, and added withwater and sieved at need, were sintered bodies which had extremelyhigher strength and were less porous as compared with the ComparativeExamples.

EXAMPLE 4

In order to examine influences of the composition of sintered bodies andthe sieve opening of the sieve after the grinding, granulated powders(Sample Nos. 42 to 55) were each obtained by forcedly drying granulatedpowder at a temperature of 80° C. for 24 hours in the same manner as inExample 3, adding 4% by weight of water thereto, and passing it througha sieve having a sieve opening of 149 μm. The thus obtained granulatedpowder was shaped and dewaxed in the same manner as in Example 3, whichwas pressurelessly sintered in a nitrogen gas atmosphere at an optimumselected firing temperature (1,600° to 1,800° C.) as giving a ratio ofβ-silicon nitride crystals being not less than 90% by weight, therebyobtaining high strength silicon nitride sintered bodies according to thepresent invention (Sample Nos. 42 to 55). Results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Producing conditions           Measurement results                                                Sieve                        Maxi-    Ratio of                                opening                                                                            Firing             Flexural                                                                           mum      crystals                                after                                                                              temper-                                                                            Composition of                                                                              strength                                                                           pore Poro-                                                                             of                  Mixing rate (wt %)  grinding                                                                           ature                                                                              sintered body (wt %)                                                                        (kg/ diameter                                                                           sity                                                                              β-Si.sub.3                                                               N.sub.4             Sample No.                                                                          Si.sub.3 N.sub.4                                                                  Y.sub.2 O.sub.3                                                                  MgO                                                                              ZrO.sub.2                                                                         (μm)                                                                            (°C.)                                                                       Si.sub.3 N.sub.4                                                                  Y.sub.2 O.sub.3                                                                  MgO                                                                              ZrO.sub.2                                                                         mm.sup.2)                                                                          (μm)                                                                            (%) (%)                 __________________________________________________________________________    Present                                                                           42                                                                              95.0                                                                              4.0                                                                              0.5                                                                              0.5 32   1800 94.8                                                                              4.1                                                                              0.5                                                                              0.5 116  8.5  0.3 99                  inven-                                                                            43                                                                              90.0                                                                              2.0                                                                              1.0                                                                              7.0 32   1750 89.7                                                                              2.0                                                                              1.0                                                                              6.8 119  7    0.4 100                 tion                                                                              44                                                                              89.0                                                                              6.0                                                                              4.0                                                                              1.0 25   1750 89.0                                                                              5.9                                                                              4.1                                                                              0.9 128  8    0.2 100                     45                                                                              87.0                                                                              6.0                                                                              4.0                                                                              3.0 20   1700 86.8                                                                              5.9                                                                              4.0                                                                              3.1 130  6    0.2 100                     46                                                                              84.0                                                                              7.0                                                                              7.0                                                                              2.0 20   1700 83.9                                                                              6.9                                                                              7.0                                                                              2.1 125  5.5  0.3 100                     47                                                                              77.0                                                                              6.0                                                                              4.0                                                                              13.0                                                                              25   1700 76.8                                                                              5.9                                                                              3.8                                                                              12.9                                                                              124  6    0.3 97                      48                                                                              74.0                                                                              15.0                                                                             4.0                                                                              7.0 25   1600 73.7                                                                              14.7                                                                             4.0                                                                              6.8 120  5    0.4 95                      49                                                                              70.0                                                                              2.0                                                                              15.0                                                                             13.0                                                                              32   1700 70.0                                                                              2.0                                                                              14.8                                                                             13.0                                                                              121  6    0.3 93                      50                                                                              75.0                                                                              6.0                                                                              16.0                                                                             3.0 32   1650 75.0                                                                              6.0                                                                              15.9                                                                             3.0 109  8    0.3 93                      51                                                                              76.0                                                                              6.0                                                                              4.0                                                                              14.0                                                                              44   1700 75.8                                                                              6.1                                                                              3.8                                                                              13.9                                                                              105  9.5  0.4 92                      52                                                                              78.0                                                                              16.0                                                                             4.0                                                                              3.0 44   1700 78.0                                                                              14.8                                                                             3.9                                                                              3.0 105  9    0.4 96                      53                                                                              85.0                                                                              1.0                                                                              7.0                                                                              7.0 20   1750 84.9                                                                              1.0                                                                              7.0                                                                              6.9 100  5.5  0.2 88                      54                                                                              89.0                                                                              7.0                                                                              4.0                                                                              0   25   1700 88.7                                                                              7.0                                                                              3.9                                                                              0   102  6    0.3 89                      55                                                                              90.0                                                                              7.0                                                                              0  3.0 25   1700 89.9                                                                              7.1                                                                              0  3.0 107  6    0.2 85                  __________________________________________________________________________

As is clear from Table 4, among the sintered bodies of the invention,those in which the granulated powder has been passed through the sieveof 32 μm or less; the mixed raw material contained from 0.5 to 15% byweight of MgO; the mixed raw material contained from 0.5 to 13% byweight of ZrO₂ ; the mixed raw material contained from 2 to 15% byweight of Y₂ O₃ ; or not less than 90% by weight of β-silicon nitridecrystals were contained in the sintered body are preferred.

In order to facilitate understanding of results in the above Examples 3and 4, FIGS. 2 and 3 show the relation between the four point flexuralstrength of sintered bodies obtained according to the present inventionand the forcedly drying temperature of granulated powders, and therelation between the four point flexural strength of the sintered bodiesand the amount of water added to the granulated powder, respectively,while FIG. 4 showing the relation between the four point flexuralstrength of the sintered bodies and the sieve opening of the sieve usedafter the grinding.

EXAMPLE 5

To α-silicon nitride powder having the average particle size of 0.5 μmwere mixed powdery MgO, ZrO₂, and Y₂ O₃ as sintering aids at rates of 4%by weight, 2% by weight, and 7% by weight, respectively. After 60% byweight of water was added to the mixture, the mixture was mixed andground by a batch grinder. The thus ground mixture was passed through asieve having a sieve opening of 20 μm, thereby obtaining a slurry of theground particles having the average particle size of 0.7 μm. Then, 2% byweight of polyvinyl alcohol (PVA) was added to the slurry, which wasconverted to a granulated powder by using a spray drier.

Further, the granulated powder was forcedly dried at a temperature shownin Table 5 for 24 hours by using a thermostatic drier, and water wasadded thereto upon necessity. When water was added, the powder wassieved by using a JIS standard sieve having a sieve opening of Table 5,thus obtaining granulated powders (Sample Nos. 61 to 69). The granulatedpowder was shaped at a pressure of 5 ton/cm² by cold isostatic press,thereby preparing a shaped body of 65 mm(φ)×50 mm(length).

Thereafter, the shaped body was dewaxed at a temperature of 500° C. for3 hours, and pressurelessly sintered at a temperature of 1,460° C. for 6hours in a nitrogen (N₂) atmosphere (primary sintering step). Then, theprimarily sintered body was treated at a temperature of 1,700° C. undera pressure of 400 atms in an N₂ atmosphere by hot isostatic press (HIP),thus obtaining sintered bodies according to the present invention(Sample Nos. 61 to 69). Apart from the above, sintered bodies (SampleNos. 70 to 72) were prepared as Comparative Examples of the presentinvention under producing conditions shown in Table 5 with no forcedlydrying.

Properties of the thus obtained sintered bodies are shown in Table 5.

Knoop hardness was measured according to JIS Z2251 after a test samplewas held under a load of 300 g for 15 seconds.

Rolling fatigue life was evaluated by cutting off a round disc of 50mm(φ)×10 mm(thick) from the sintered sample, mirror polishing thesurface of the round disc, and subjecting it to a rolling fatigue testat a hertz stress of 500 kg/mm² in a six ball type thrust bearingtester.

As is clear from Table 5, the HIP sintered bodies according to thepresent invention using the mixed raw material being forcedly dried, andadded with water and further sieved at need, are sintered bodies whichare extremely less porous and have excellent mechanical strengths, ascompared with Comparative Examples.

                                      TABLE 5                                     __________________________________________________________________________           Producing conditions Measurement results                                      Forcedly                                                                             Amount                                                                             Sieve opening                                                                         Maximum         Rolling                                   drying of water                                                                           after water                                                                           pore       Knoop                                                                              fatigue                                   temperature                                                                          added                                                                              added   diameter                                                                            Porosity                                                                           hardness                                                                           life                               Sample No.                                                                           (°C.)                                                                         (wt %)                                                                             (μm) (μm)                                                                             (%)  (GPa)                                                                              (cycle)                            __________________________________________________________________________    Present                                                                            61                                                                              40     --   --      8.0   0.25 15.5 1.5 × 10.sup.7               inven-                                                                             62                                                                              60     --   --      5.0   0.19 16.1 6 × 10.sup.7                 tion 63                                                                              60     0.5  149     3.5   0.12 16.7 40 × 10.sup.7                     64                                                                              70     3    325     1.0   0.01 17.2 >110 × 10.sup.7                   65                                                                              80     5    250     2.5   0.02 17.0 >80 × 10.sup.7                    66                                                                              90     --   --      4.0   0.10 16.5 30 × 10.sup.7                     67                                                                              100    3    325     4.0   0.09 16.5 35 × 10.sup.7                     68                                                                              120    6    250     5.5   0.20 15.8 9 × 10.sup.7                      69                                                                              140    5    149     7.0   0.23 15.7 3 × 10.sup.7                 Compar-                                                                            70                                                                              --     3    149     11.5  0.43 14.5 0.4 × 10.sup.7               ative                                                                              71                                                                              --     4    --      13.0  0.42 14.7 0.25 × 10.sup.7              example                                                                            72                                                                              --     --   --      16.0  0.58 13.8 0.07 × 10.sup.7              __________________________________________________________________________

EXAMPLE 6

In order to examine influences of sieving after grinding and the averageparticle size after the grinding, except that the kind and the additionamount of sintering aids were changed as shown in Table 6, in the samemanners as in Example 5, the granulated powder was forcedly dried at 80°C. for 24 hours, 4% by weight of water was added thereto, and the powderwas passed through a sieve having a sieve opening of 149 μm, thusobtaining granulated powders (Sample Nos. 73 to 77).

After the thus obtained granulated powder was shaped and dewaxed in thesame manner as in Example 5, the powder was sealed in a silica glasscapsule under vacuum. Then, the capsule was placed in a HIP apparatus,which was HIP-treated at a temperature of 1,600° C. under a pressure of1,500 atms, thus obtaining silicon nitride sintered bodies (Sample Nos.73 to 77). With respect to the thus obtained sintered bodies, rollingfatigue test was effected similarly as in Example 5, and further, a wearresisting test was effected.

In the wear resisting test, a cylindrical sample of 15 mm(φ)×15mm(length) was cut off from each of Sample Nos. 73 to 77, and abradedwith a #140 diamond grind stone, which was subjected to the wearresisting test by using a ball mill. Test conditions were that analumina vessel having an inner diameter of 120 mm(φ) was used, androtated at 120 rpm.

A slurry liquid in which #100 silicon carbide powder and water weremixed at a weight ratio of 1:1 was filled up to a half of the vessel.Then, five of the above-prepared samples of 15 mm(φ)×15 mm(length) wereplaced in the slurry, and then subjected to the wear resisting test for24 hours.

A wear amount was determined from changes in weight and dimension beforeand after the test. Results in the rolling fatigue test and the wearresisting test are shown in Table 6.

It is seen from Table 6 that among the invention products, those inwhich the ground powder had been passed through the sieve of not morethan 32 μm, or the average particle size after the grinding was not morethan 1 μm are preferred.

                                      TABLE 6                                     __________________________________________________________________________    Producing conditions                                                                            Sieve Average                                                                             Measurement results                                               opening                                                                             particle                                                                           Maximum         Rolling                                            after water                                                                         size after                                                                         pore       Wear fatigue                                Composition added grinding                                                                           diameter                                                                            Porosity                                                                           amount                                                                             life                             Sample No.                                                                          (wt %)      (μm)                                                                             (μm)                                                                            (μm)                                                                             (%)  (kg/cm.sup.2)                                                                      (cycle)                          __________________________________________________________________________    Present                                                                           73                                                                              92% Si.sub.3 N.sub.4 --2% SrO--                                                           44    0.7  9.0   0.30 0.10  2 × 10.sup.7              inven-                                                                              3% MgO--3% CeO.sub.2                                                    tion                                                                              74                                                                              92% Si.sub.3 N.sub.4 --2% SrO--                                                           32    "    4.0   0.11 0.03 50 × 10.sup.7                    3% MgO--3% CeO.sub.2                                                        75                                                                              92% Si.sub.3 N.sub.4 --2% SrO--                                                           17    "    3.0   0.06 0.01 70 × 10.sup.7                    3% MgO--3% CeO.sub.2                                                        76                                                                              91% Si.sub.3 N.sub.4 --5% Y.sub.2 O.sub.3 --                                              25    0.8  5.0   0.15 0.06 30 × 10.sup.7                    4% Al.sub.2 O.sub.3                                                         77                                                                              91% Si.sub. 3 N.sub.4 --5% Y.sub.2 O.sub.3 --                                             "     1.4  7.5   0.25 0.09  5 × 10.sup.7                    4% Al.sub.2 O.sub.3                                                     __________________________________________________________________________

EXAMPLE 7

To a silicon nitride raw powder having an average particle size of 0.7μm and containing 97% by weight of α-silicon nitride were mixed powderyMgO, SrO, and CeO₂ at rates of 3.5% by weight, 1.5% by weight, and 5% byweight, respectively. After 60% by wight of water was added to themixture, the mixture was mixed and ground by a batch grinder and thenpassed through a sieve having a sieve opening of 20 μm, therebyobtaining a slurry containing powder having the average particle size of0.5 μm. Then, 2% by weight of polyvinyl alcohol (PVA) was added to theslurry, which was converted to a granulated powder by using a spraydrier.

Further, the granulated powder was forcedly dried at a temperature shownin Table 7 for 24 hours by using a thermostatic drier, and uponnecessity, wear was added and the mixture was sieved through a JISstandard sieve having a sieve opening shown in Table 7, thus obtaininggranulated powders (Sample Nos. 81 to 88). The granulated powder wasshaped at a pressure of 3 ton/cm² by cold isostatic press, therebypreparing a shaped body of 65 mm(φ)×50 mm(length).

Thereafter, the shaped body was dewaxed at a temperature of 500° C. for3 hours, and the pressurelessly sintered at a temperature of 1,460° C.for 6 hours in a nitrogen (N₂) atmosphere (primary sintering step).Then, the primarily sintered body was treated by a hot isostatic press(HIP) at a temperature of 1,680° C. under a pressure of 400 atms in anN₂ atmosphere, thus obtaining sintered bodies according to the presentinvention (sample Nos. 81 to 88). Apart from the above, sintered bodies(Sample Nos. 89 to 91) were prepared as comparative Examples of thepresent invention under producing conditions shown in Table 7 with noforcedly drying.

Properties of the thus obtained sintered bodies are shown in Table 7.

Flexural strength, maximum pore diameter, and porosity of the thusobtained sintered bodies were measured in the same manner as in Example1, and measurement results are shown in Table 7.

As is clear from Table 7, the HIP sintered bodies according to thepresent invention using the mixed raw material having been forcedlydried, and added with water and sieved at need, are sintered bodieswhich are far less porous and have excellent mechanical strength, ascompared with Comparative Examples.

                                      TABLE 7                                     __________________________________________________________________________           Producing conditions                                                                              Measurement results                                       Forcedly                                                                             Amount                                                                             Sieve opening                                                                         maximum    4-point                                        drying of water                                                                           after water                                                                           pore       flexural                                       temperature                                                                          added                                                                              added   diameter                                                                            Porosity                                                                           strength                                Sample No.                                                                           (°C.)                                                                         (wt %)                                                                             (μm) (μm)                                                                             (%)  (kg/mm.sup.2)                           __________________________________________________________________________    Present                                                                            81                                                                              40     --   --      9.0   0.35 121                                     inven-                                                                             82                                                                              60     0.5  149     4.5   0.17 125                                     tion 83                                                                              70     3    325     2.0   0.02 131                                          84                                                                              80     5    250     3.5   0.03 135                                          85                                                                              90     --   --      5.0   0.15 126                                          86                                                                              100    3    325     4.5   0.05 140                                          87                                                                              120    6    250     6.5   0.25 125                                          88                                                                              140    5    149     8.0   0.25 122                                     Compar-                                                                            89                                                                              --     3    149     12.0  0.44  96                                     ative                                                                              90                                                                              --     4    --      13.5  0.43  95                                     example                                                                            91                                                                              --     --   --      16.5  0.60  92                                     __________________________________________________________________________

EXAMPLE 8

In order to examine influences of the sieving after the grinding and theaverage particle size after the grinding, an α-silicon nitride rawmaterial (containing 99.5% by weight of α-silicon nitride) was mixedwith sintering aids of which kinds and mixing ratios are shown in Table8. After 60% by weight of water was added to the mixture, the mixturewas ground and sieved at a sieve opening as shown in Table 8, therebyobtaining a slurry. The average particle size in the slurry was shown inTable 8. Then, 1% by weight of polyvinyl alcohol and 0.5% by weight ofstearic acid were added to the thus obtained slurry, which was convertedto a granulated powder by using a spray drier. After the granulatedpowder was dried at 70° C. for 20 hours by using a drier and 4% byweight of water was added thereto, the powder was passed through a sievehaving a sieve opening of 149 μm. The resulting powder was shaped at apressure of 6 ton/cm² by cold isostatic press, thereby obtaining ashaped body of 30 mm(φ)×60 mm(length). Then, the shaped body was dewaxedat 500° C. for 3 hours, and then sealed in a silica glass capsule undervacuum. Next, the capsule was inserted into a HIP apparatus, andHIP-treated at a temperature of 1,700° C. under a pressure of 1,500atms, thus obtaining silicon nitride sintered bodies (Sample Nos. 92 to97).

As is clear from Table 8, among the invention products, those in whichthe powder had been passed through the sieve of a sieve opening of notmore than 32 μm after the grinding, or the average particle size afterthe grinding was not more than 1 μm are preferred.

                                      TABLE 8                                     __________________________________________________________________________                          Sieve                                                                         opening                                                                            Average particle                                                                       Maximum    4-point                                              after                                                                              size after                                                                             pore       flexural                       Sample                                                                            Mixing rate (wt %)                                                                              grinding                                                                           mixing and                                                                             diameter                                                                            Porosity                                                                           strength                       No. Si.sub.3 N.sub.4                                                                  Y.sub.2 O.sub.3                                                                  MgO                                                                              ZrO.sub.2                                                                         CeO.sub.2                                                                         (μm)                                                                            grinding (μm)                                                                       (μm)                                                                             (%)  (kg/mm.sup.2)                  __________________________________________________________________________    92  91  5  3  1   0   44   0.6      8.0   0.20 127                            93  "   "  "  "   "   25   "        3.5   0.08 145                            94  "   "  "  "   "   17   "        2.5   0.06 157                            95  88  5  5  0   2   32   0.4      5.0   0.11 135                            96  "   "  "  "   "   "    0.9      7.5   0.25 123                            97  "   "  "  "   "   "    1.4      8.0   0.30 120                            __________________________________________________________________________

EXAMPLE 9

To an α-silicon nitride raw material containing 85% by weight, 92% byweight, or 95% by weight of α-silicon nitride were added 6% by weight ofY₂ O₃ and 4% by weight of Al₂ O₃ as sintering aids, which were mixed,ground, and sieved by a sieve having a sieve opening of 32 μm, therebyobtaining a slurry. Then, 1% by weight of methyl cellulose was added tothe slurry, which was dried by a drier to prepare a granulated powder.The granulated powder was further dried by a thermostatic drier at 100°C. for 30 hours, and shaped at a pressure of 3 ton/cm² by cold isostaticpress, thereby obtaining a shaped body of 30 mm(φ)×50 mm(length). Then,the shaped body was dewaxed at 450° C. for 5 hours and sealed in asilica glass capsule under vacuum. Next, the capsule was inserted into aHIP apparatus, and HIP-treated at a temperature of 1,600° , 1,650,° or1,700° C. under a pressure of 1,000 atms for 30 minutes, thus obtainingsilicon nitride sintered bodies (Sample Nos. 98 to 104) as shown inTable 9.

                  TABLE 9                                                         ______________________________________                                                  Content of                                                                    α-silicon                                                                         Treating Content of                                                                             4-point                                           nitride in                                                                              temper-  β-silicon                                                                         flexural                                          raw       ature    nitride in                                                                             strength                                          material  in HIP   sintered body                                                                          (kg/                                    Sample No.                                                                              (%)       (°C.)                                                                           (%)      mm.sup.2)                               ______________________________________                                        Example                                                                               98    95        1600   88       129                                           99              1650   93       135                                          100              1700   100      141                                          101    92        1650   93       123                                          102              1700   100      127                                   Compar-                                                                              103    85        1650   95       104                                   ative  104              1700   100      105                                   example                                                                       ______________________________________                                    

As is clear from Table 9, it was revealed that the sintered bodies usingthe silicon nitride raw material containing not less than 90% by weightof α-silicon nitride have higher strength.

As is clear from the forgoing, according to the present invention, highstrength, high density and/or high hardness fine ceramic sintered bodieshaving excellent mechanical strength with smaller maximum pore diameterand porosity can industrially be obtained at an inexpensive cost due toa synergistic effect of forcedly drying of the granulated powder, andwater incorporation and/or sieving at need, irrespective of the HIPtreatment and the pressureless sintering. Consequently, the ceramicsintered bodies according to the present invention can be applied to,for instance, high temperature bearings, engine parts, gas turbineparts, and the like, and thus have extremely great industrial value.

What is claimed is:
 1. A process for producing a silicon nitridesintered body, comprising:mixing a silicon nitride raw material powderwith a sintering aid selected from the group consisting of a compound ofMg, Zr and Y, which forms MgO, ZrO₂ and Y₂ O₃, respectively, uponheating; granulating said resultant mixture to form a granulated powder;forcibly drying said granulated powder; shaping said dried, granulatedpowder to form a shaped body; and firing the shaped body.
 2. A processaccording to claim 1, wherein said granulated powder is forcibly driedat a temperature of 60°-100° C.
 3. A process according to claim 1,further comprising the step of passing the mixture through a sievehaving a sieve opening of not greater than 32 microns, before saidmixture is granulated.
 4. A process according to claim 1, furthercomprising the step of sieving the dried, granulated powder through asieve having a sieve opening of not greater than 250 microns.
 5. Aprocess according to claim 1, further comprising the steps of addingwater to the dried, granulated powder in an amount not greater than 6.0wt % and then sieving the dried, granulated powder/water mixture througha sieve having a sieve opening of not greater than 250 microns.
 6. Aprocess according to claim 5, wherein said water is present in an amountbetween 0.5-5.0 wt %.
 7. A process according to claim 1, wherein theaverage particle size of the silicon nitride raw material powder aftermixing and grinding is not greater than 1 micron.
 8. A process accordingto claim 1, wherein the firing step consists of pressureless sintering.9. A process according to claim 1, wherein the granulating step consistsof spray drying.
 10. A process according to claim 9, wherein said spraydrying step includes the use of at least one auxiliary material selectedfrom the group consisting of polyvinyl alcohol, polyethylene glycol,methyl cellulose, stearic acid and combinations thereof.
 11. A processaccording to claim 1, wherein the average particle size of the siliconnitride raw material powder is not greater than 2 microns.
 12. A processaccording to claim 1, wherein said silicon nitride raw material powdercontains not less than 90 wt % of α-silicon nitride.
 13. A processaccording to claim 1, further comprising the steps of subjecting theshaped body to a preliminary treatment selected from the groupconsisting of a primary firing at a temperature between 1400°-1600° C.in a nitrogen atmosphere and sealing the shaped body in a capsule, andthen hot isostatically pressing the preliminarily treated body in aninert atmosphere.
 14. A process according to claim 13, wherein the bodyconsists mainly of silicon nitride.
 15. A process for producing a ZrO₂sintered body, comprising:mixing a ZrO₂ raw material powder with asintering aid selected from the group consisting of a compound of Mg, Yand Al, which forms MgO, Y₂ O₃ and Al₂ O₃, respectively, upon heating;grinding a resultant mixture of said ZrO₂ raw material powder and saidsintering aid; granulating said resultant mixture to form a granulatedpowder; forcibly drying said granulated powder; shaping said dried,granulated powder to form a shaped body; and firing the shaped body. 16.A process according to claim 15, wherein said granulated powder isforcibly dried at a temperature of 60°-100° C.
 17. A process accordingto claim 15, further comprising the step of passing the mixture througha sieve having a sieve opending of not greater than 32 microns, beforesaid mixture is granulated.
 18. A process according to claim 15, furthercomprising the step of sieving the dried, granulated powder through asieve having a sieve opening of not greater than 250 microns.
 19. Aprocess according to claim 15, further comprising the steps of addingwater to the dried, granulated powder in an amount not greater than 6.0wt % and then sieving the dried, granulated powder/water mixture througha sieve having a sieve opening of not greater than 250 microns.
 20. Aprocess according to claim 19, wherein said water is present in anamount between 0.5-5.0 wt %.
 21. A process according to claim 15,wherein the average particle size of the ZrO₂ raw material powder aftermixing and grinding is not greater than 1 micron.
 22. A processaccording to claim 15, wherein the firing step consists of pressurelesssintering.
 23. A process according to claim 15, wherein the granulatingstep consists of spray drying.
 24. A process according to claim 23,wherein said spray-drying step includes the use of at least oneauxiliary material selected from the group consisting of polyvinylalcohol, polyethylene glycol, methyl cellulose, stearic acid andcombinations thereof.
 25. A process according to claim 18, wherein theaverage particle size of the ZrO₂ raw material powder is not greaterthan 2 microns.
 26. A process according to claim 15, further comprisingthe steps of subjecting the shaped body to a preliminary treatmentselected from the group consisting of a primary firing at a temperaturebetween 1400°-1600° C. in a nitrogen atmosphere and sealing the shapedbody in a capsule, and then hot isostatically pressing the preliminarilytreated body in an inert atmosphere.